EP0421507B1

EP0421507B1 - Method of manufacturing a bipolar transistor

Info

Publication number: EP0421507B1
Application number: EP90202437A
Authority: EP
Inventors: Wilhelmus Jacobus Maria Joseph Josquin; Jan Van Dijk
Original assignee: Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1989-09-20
Filing date: 1990-09-14
Publication date: 1996-07-03
Anticipated expiration: 2010-09-14
Also published as: KR910007162A; GB2236901A; US5023192A; GB8921262D0; EP0421507A3; JPH03112136A; DE69027644T2; EP0421507A2; DE69027644D1; JPH0727915B2

Description

This invention relates to a method of manufacturing a semiconductor device such as, for example, a bipolar transistor.
US-A-4 772 566 discloses a method of manufacturing a semiconductor device, which method comprises providing a semiconductor body having adjacent one major surface a first region of one conductivity type, providing on the one major surface a doped layer from which impurities diffuse into the semiconductor body to form an extrinsic subsidiary region of a second region of opposite conductivity type within the first region, forming an opening through the doped layer and over the first region, defining a first portion of an insulating material on the side wall of the doped layer bounding the opening and a second portion of a different material on the first portion, and introducing through the opening impurities for forming an intrinsic subsidiary region of the opposite conductivity type second region within the first region and a third region of the one conductivity type within the intrinsic subsidiary region.
The method described in US-A-4 772 566 relates to the manufacture of a bipolar transistor in which the first region comprises the collector region of the transistor and in which the second and third regions form the base and emitter regions, respectively.
It should be understood that as used herein the term 'intrinsic subsidiary region' refers to the active area of the second region, that is in the case of the bipolar transistor described in US-A-4 772 566 the active base region, whilst the term 'extrinsic subsidiary region' refers to that area which couples to the 'intrinsic subsidiary region' to facilitate contact to the second region.
As described in US-A-4 772 566, the doped layer is provided as a doped polycrystalline layer, for example a doped polycrystalline silicon layer. After definition of the polycrystalline layer to form the opening through which the impurities are to be introduced to form the intrinsic base and the emitter regions and to form a collector contact opening, a thin thermal oxide layer is grown on sthe polycrystalline layer. With the base and emitter opening masked, impurities are introduced to form a highly doped collector contact region. After formation of the highly doped collector contact region, this opening is masked and impurities for forming the intrinsic base region are then implanted. A conformal dielectric, for example silicon dioxide, layer is then deposited and covered by a polycrystalline layer, again this may be a polycrystalline silicon layer. The polycrystalline layer is then anisotropically etched to define the second portion and the dielectric layer then etched, using the polycrystalline second portion as a mask, to define the insulating first portion. A polycrystalline silicon layer is then deposited and doped with impurities of the one conductivity type which are subsequently caused to diffuse into the underlying semiconductor body to form the emitter region.
The method described in US-A-4 772 566 employs a composite insulating spacer formed by the first and second portions which enables good separation of the emitter region and extrinsic base region to avoid hot carrier effects which could result in low emitter-base breakdown voltages. Also, the loss of dopant into the polycrystalline silicon reduces the geometry dependence of the transistor parameters. However, the etching of the composite spacer needs to be carried out with a process which enables the first and second portions to be etched with a high selectivity relative to the semiconductor body because otherwise a proportion of the impurities introduced for forming the intrinsic subsidiary region will be lost during the etching process. Anisotropic etching processes enable good control of the formation of the first and second portions but do not provide very high selectivity whilst wet etching processes although enabling high selectivity may result in underetching of the second portion so that the dimensions are not so well controlled.
It is an object of the present invention to provide a method of manufacturing a semiconductor device in which the separation or offset of the intrinsic subsidiary region and the third device region is well controlled to assist in avoiding hot carrier effects and to reduce, in the case of a bipolar transistor, the dependence of current amplification on the width, that is by convention the dimension of the emitter region measured in a direction parallel to the one major surface, of the emitter region.
According to the present invention there is provided a method of manufacturing a semiconductor device having first, second and third regions, which method comprises providing a semiconductor body having in one major surface the first region of having a first conductivity type, providing on the one major surface a layer doped with impurities of the opposite conductivity tyre, and, forming an opening through the doped layer over the first region, causing impurities to diffuse from the doped layer into the semiconductor body thus forming an extrinsic subsidiary region of the second region within the first region said extrinsic subsidiary region thus being the opposite conductivity type, introducing through the opening impurities to form a coupling region of the opposite conductivity type within the first region and joining with the extrinsic subsidiary region, providing a first layer of insulating material over the opening, etching the insulating material to leave first portions of the insulating material on the side walls of the doped layer bounding the opening so that the first portions define a first window smaller than the opening, introducing impurities for forming an intrinsic subsidiary region of the second region through the first window said second region thus comprising the extrinsic subsidiary region, the coupling region and the intrinsic subsidiary region with the coupling region joining the extrinsic subsidiary region to the intrinsic subsidiary region, providing a second layer of a material different from the first layer over the first window, etching the second layer selectively with respect to the first portions to define on the insulating first portions second portions of the different material, which second portions define a second window smaller than the first window, and introducing impurities of the first conductivity type through the second smaller window to form the third region within the intrinsic subsidiary region.
Thus, using a method in accordance with the invention, the impurities for forming the intrinsic subsidiary region, for example the intrinsic base region of a bipolar transistor, are introduced via a first window defined by the insulating first portion whilst the impurities for forming the third region (which will form the emitter region in the case of a bipolar transistor) are introduced via a second smaller window defined by the different material second portion enabling the third region to be well spaced from the extrinsic subsidiary region so as to inhibit hot carrier effects. In addition, before defining the insulating first portion, impurities are introduced through the opening for forming a coupling region of the opposite conductivity type for ensuring connection of the extrinsic and intrinsic subsidiary regions. This coupling region enables the extrinsic subsidiary region and the third region to be well spaced so as to inhibit edge effects and leakage currents to avoid or at least inhibit possible punch-through between the first and third regions. Generally, the impurities for forming the coupling region are implanted using a dose and energy such that the coupling region is more shallow and lowly doped than the intrinsic subsidiary region of the second region so that any variations in the definition of the first and second portions do not significantly affect the width (that is be convention the depth into the semiconductor body where the intrinsic subsidiary region is the intrinsic base region of a bipolar transistor) of the intrinsic subsidiary region of the second region.
The insulating first portion may be provided by anisotropically etching the insulating first layer provided over the opening and the second portion by anisotropically etching the second layer of a material selectively with respect to the insulating first portion. The second portion may also be an insulating portion. The insulating first portion may be defined by depositing an oxide, for example a silicon oxide, layer over the doped layer and then anisotropically etching the oxide layer to leave the insulating first portion whilst the second portion may be defined by depositing a polycrystalline, for example a polycrystalline silicon, layer over the doped layer and then anisotropically etching the polycrystalline layer to leave the second portion. A further insulating layer, for example a silicon oxide layer, may be provided over the first window prior to the second layer so facilitating etching of the second portion and enabling the second portion, when defined, to be spaced from the surface of the semiconductor body exposed in the opening by the further insulating layer. With such a method, the width, that is the depth into the semiconductor body, of the intrinsic subsidiary region, the intrinsic base region in the case of a bipolar transistor, is less sensitive to variations in the etching of the insulating material forming the insulating first portion. In addition, especially in the case of a bipolar transistor, the control of the separation of the intrinsic base and emitter regions which this allows enables the current amplification to be much less dependent on the width (that is the dimension along the one major surface) of the emitter region so that the current amplification is not significantly degraded by reduction in the device dimensions.
Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 and 2 are cross-sectional views of part of a semiconductor body for illustrating steps in a method in accordance with the invention for manufacturing a bipolar transistor;
Figures 3 to 7 are enlarged cross-sectional views of a portion of the semiconductor body shown in Figures 1 and 2 for illustrating further steps of a method in accordance with the invention for manufacturing a bipolar transistor; and
Figure 8 is a cross-sectional view showing a bipolar transistor manufactured using a method in accordance with the invention.

It should, of course, be understood that the Figures are merely schematic and are not drawn to scale various dimensions, such as the thickness of layers, having been exaggerated relative to other dimensions in the interests of clarity.
Referring now to the drawings, there is illustrated a method of manufacturing a semiconductor device having first, second and third regions, which method comprises providing a semiconductor body 10 having in one major surface 11 the first region 20 having a first conductivity type, providing on the one major surface 11 a layer (30) doped with impurities of the opposite conductivity type, and, forming an opening 31 through the doped layer 30 over the first region 20, causing impurities to diffuse from the doped layer 30 into the semiconductor body 10 thus forming an extrinsic subsidiary region 41 of the second region 40 within the first region 20 said extrinsic subsidiary region (41) thus being the opposite conductivity type, introducing through the opening 31 impurities to form a coupling region 43 of the opposite conductivity type within the first region 20 and joining with the extrinsic subsidiary region 41, providing a first layer 5 of insulating material over the opening 30, etching the insulating material to leave first portions 50 of the insulating material on the side walls 32 of the doped layer 30 bounding the opening 31 so that the first portions 50 define a first window 80 smaller than the opening 30, introducing impurities for forming an intrinsic subsidiary region 42 of the second region 40 through the first window 80 said second region 40 thus comprising the extrinsic subsidiary region 41, the coupling region 43 and the intrinsic subsidiary region 42 with the coupling region 43 joining the extrinsic subsidiary region 41 to the intrinsic subsidiary region 42, providing a second layer 6 of a material different from the first layer 5 over the first window 80, etching the second layer 6 selectively with respect to the first portion 50 to define on the insulating first portions 50 second portions 60 of the different material which second portions 60 define a second window 90 smaller than the first window 80, and introducing impurities of the first conductivity type through the second smaller window 90 to form the third region 70 within the intrinsic subsidiary region 42.
Turning now to the specific example shown in the drawings, there is illustrated a method in accordance with the invention of manufacturing a bipolar transistor which may be suitable for integration within a BiCMOS process, that is a process in which complementary n- and p- channel insulated gate field effect transistors (IGFETs) are formed together with bipolar transistors in the same semiconductor body or may be used in a purely bipolar process.
In the example to be described below, the semiconductor body 10 comprises a monocrystalline silicon substrate 1 of the opposite conductivity type, in this case p- conductivity type, with a resistivity of, typically, 10 ohm-cm into which impurities of the one conductivity type (n- conductivity type in this example) are introduced using an appropriate mask to form at a device area 2 a highly doped region 21 which will later form part of the collector region 20 of the bipolar transistor. A layer 3 (indicated by dashed lines in the Figures) of p- conductivity type silicon with a resistivity of, typically, 8 to 12 ohm-cm is epitaxially grown on the substrate 1, thereby burying the highly doped region 21.
Using conventional photolithographic and etching techniques a mask layer (not shown) is then defined and impurities are introduced, in this case phosphorus ions are implanted, to form an n conductivity type region 22 directly above and contacting the buried region 21. The region 22 forms the main subsidiary region of the collector region 20.
The device area 2 is then defined or separated from other device areas (not shown) of the semiconductor body 10 by forming field oxide regions 4 using, for example, conventional local oxidation of silicon techniques with a silicon oxide-silicon nitride anti-oxidation mask (not shown).
At this stage various threshold adjustment implantations may be carried out if necessary. Also, although not shown, highly conductive channel stopper regions may be formed by ion implantation beneath the field oxide regions 4. As shown in Figure 1, in addition to defining the device area 2, the field oxide regions 4a, 4b serve to define a collector contact area 23 which is doped twice with impurities of the one conductivity type, once during formation of the main subsidiary region 22 of the collector region 20 and then again through an appropriate mask to form a highly doped contact region to enable ohmic contact to the collector region 20.
A dopable layer 300, usually a semiconductor layer and, in this example a polycrystalline silicon layer, is then deposited using conventional low pressure chemical vapour deposition techniques (LPCVD). P conductivity type, in this example boron ions, are then implanted into the polycrystalline silicon over the device area 2.
An insulating layer 35, for example a layer of silicon dioxide, is then deposited over the device area 2.
A conventional mask (not shown) is then defined over the insulating layer 35 and the insulating layer 35 and the doped polycrystalline silicon layer are then patterned using conventional techniques to define the doped layer 30 and to form the opening 31 thereby producing the structure shown in Figure 2.
P conductivity type ions, in this example boron ions, are then implanted using a low dose and energy for forming the coupling region 43 of the second region 40. The coupling region 43 is a shallow region and, typically, boron (B+) ions with an energy of about 10keV (kilo-electron volts) and a dose of in the range of from about 10¹² to about 10¹³ ions cm^-2 are used. The purpose of this shallow coupling region 43 will be explained below.
After patterning of the insulating layer 35 and the doped layer 30 a thin thermal oxide layer 33 is grown on the exposed silicon surface regions. An insulating layer 5, in this example a layer of tetra-ethyl-ortho-silicate (TEOS), is then deposited over the structure as shown in Figure 3. By this stage in the process there may have been a certain amount of diffusion of boron ions out of the doped layer 30 into the semiconductor body 1 forming a precursor region 41a which will eventually provide the extrinsic region 41 of the second region 40. The insulating layer 5 is then etched anisotropically using conventional plasma etching (eg a CF₄ or CHF₃+O₂ plasma) techniques to leave the insulating first portion or spacer 50 on the side wall 32 of the doped layer 30 bounding the opening 31 as shown in Figure 4. If the desired width of the third device or emitter region 70 is about 0.4µm (micrometres) and the opening 31 is about 1.0µm, then the thickness of the insulating layer 50 will be selected such that the insulating first portion or spacer 50 is about 0.2µm wide (or thick) at its widest point, that is adjacent the exposed surface area 11a.
During the anisotropic etching to form the first insulating portion 50, a small amount of the exposed surface area 11a of the semiconductor body 1, will be lost or removed and some or possibly even all of the boron ions for forming the shallow subsidiary region 43 implanted at this exposed surface area 11a will be lost or removed. A small amount of the boron ions may however remain.
After formation of the insulating first portion 50 as shown in Figure 4, a thin further insulating layer 36, for example a thermal oxide layer with a thickness of about 25nm (nanometres), is provided over the surface and, in this example, p conductivity type impurities are introduced to provide, as shown in Figure 5, a precursor region 42a for eventually forming the intrinsic subsidiary region 42 which will in this example, form the intrinsic base region of the bipolar transistor. A conventional masking layer (not shown) is used to mask the collector contact region 23 from this implantation. In the example being described, the p conductivity type impurities are introduced by implanting boron (B+) ions with an energy of about 35keV and a dose of in the range of about 2 to 5x10¹³ ions cm^-2. As an alternative, the p conductivity type impurities for forming the precursor region 42a may be introduced prior to forming the further insulating layer 36. In this case, the further insulating layer 36 may be a deposited layer, for example a TEOS layer.
A layer 6 of undoped (that is not intentionally doped) polycrystalline material, in this example a layer of undoped polycrystalline silicon of about 0.15µm thickness, is then deposited, using conventional low pressure chemical vapour deposition techniques, over the thin further insulating layer 36.
The polycrystalline silicon layer 6 is then etched using an anisotropic process, for example a chlorine plasma etching process, which etches polycrystalline silicon with high selectivity with respect to insulating material such as, in this example, a thermal oxide or TEOS. This anisotropic etching of the undoped polycrystalline silicon layer 6 results in a small, for example 0.1µm for the emitter dimensions given above, undoped polycrystalline silicon spacer which forms the second portion 60 on the first portion 50 as shown in Figure 6.
The thin further insulating layer 36 masks the semiconductor surface region 11a from the etchant used to etch the polycrystalline silicon layer 6 enabling the polycrystalline silicon layer 6 to be etched with high selectivity whilst avoiding erosion by the etchant of the silicon surface region 11a. This means that the width, that is by convention the depth into the semiconductor body, of the intrinsic base region 42 is less sensitive to the etching processes used to form the first and second portions 50 and 60. Also, the remaining portion of the insulating layer 36 provides a barrier beneath the polycrystalline silicon second portion 60 to prevent diffusion of the p type impurities out of the semiconductor body into the polycrystalline silicon second portion 60.
The undoped polycrystalline silicon second insulating portion 60 thus defines the second smaller window 90 through which n-conductivity type impurities may be introduced to form the third device, in this case the emitter, region 70.
The thin further insulating layer 36 covering the surface area 11a is then removed leaving only the portion 36a beneath the polycrystalline silicon second insulating portion 60. The thin further insulating 36 can be etched with a high selectivity relative to the silicon surface region 11a, for example by using a plasma etching process such as that used to form the first portion 50, so that erosion of the surface region 11a and thus loss of impurities introduced to form the intrinsic subsidiary region 42 is avoided or at least reduced.
A dopable layer 7, in this example another layer of polycrystalline silicon with a thickness of about 150nm, is then deposited by conventional LPCVD techniques in the second smaller window 90 and doped with n conductivity type impurities, in this case by implantation of arsenic (As⁺) ions with a dose of about 7.5x10¹⁵ ions cm^-2 at an energy of about 75keV. After patterning of the doped polycrystalline silicon layer 7 using conventional photolithographic and etching techniques to leave the layer 7 only covering the desired area as shown in Figure 7, the semiconductor body is heated, for example to about 925°C (degrees Celsius) for about 60 minutes in a nitrogen atmosphere, to cause n conductivity type impurities to diffuse out of the doped layer 7 into the semiconductor body 1 to form the third, that is in this example the emitter, region 70. This heating treatment also drives in the previously introduced impurities so as to form the extrinsic and intrinsic subsidiary regions 41 and 42 of the second, that is in this example the base, region 40. Alternatively, instead of forming the emitter region 70 by causing impurities to diffuse out of a doped layer, the impurities for forming the emitter region 70 could simply be implanted.
The remaining portion of the thin further insulating layer 36a beneath the polycrystalline silicon second portion 60 serves to prevent or inhibit impurities in the doped layer 7 (or impurities being implanted to form the emitter region 70) passing through the second portion 60 into the semiconductor body 1.
The shallow coupling region 43 is more shallow and lowly doped than the intrinsic subsidiary region 42 and is overdoped by the n conductivity type impurities introduced for forming the emitter region 70. However, outside the area of the emitter region 70, the coupling region 43 serves to ensure that there is a good low resistance coupling between the extrinsic and intrinsic subsidiary regions 41 and 42. The coupling region 43 thus enables the extrinsic subsidiary region 41 to be well spaced from the emitter region 70 so as to reduce leakage currents and the possibility of parasitic bipolar action whilst avoiding or at least inhibiting the possibility of punch-through between the emitter and collector regions 70 and 20.
After formation of the emitter region 70 as shown in Figure 7, a further masking layer (not shown) is applied to allow the opening of contact holes for enabling a first level of metallisation, for example a layer of titanium-tungsten alloy followed by a layer of silicon-containing aluminium, to be deposited and patterned by conventional techniques to form collector, base and emitter contact electrodes C,B and E as shown in Figure 8. In order to improve ohmic contact to the electrodes, a silicide, for example a cobalt or titanium silicide, layer may be formed in conventional self-aligned manner on the exposed silicon surfaces before deposition of the metallisation.
In the method described above, the impurities for forming the intrinsic base region 42 are introduced through the first window 80 defined by the insulating first portion 50 whilst the impurities for forming the emitter region are introduced through the second smaller window 90 defined by the second portion 60 which is formed by an etching process which is selective with respect to the insulating first portion 50. This enables the emitter region 70 to be well spaced from the extrinsic base region 41 so as to inhibit hot carrier effects. Also, as the second portion 60 is defined more independently of the insulating first portion 50, the width (that is, by convention, the depth into the semiconductor body in the case of the intrinsic base region of a bipolar transistor) of the intrinsic subsidiary region 42 can be controlled more accurately and is less susceptible to over-etching of the oxide forming the insulating first portion 50. In addition, especially in the case of a bipolar transistor, the control of the separation of the intrinsic base and emitter regions which this allows enables the current amplification (hfe) to be much less dependent on the width (that is, by convention, the dimension parallel to the one major surface 11) of the emitter region so that the current amplification is not significantly degraded by reduction in the device dimensions. In particular, the inventors have found that, using the method described above, a bipolar transistor having a measured emitter width of about 0.4µm has a current amplification which is not significantly degraded with respect to that of a bipolar transistor having an emitter width of about 10µm manufactured using the same method.
Although in the method described above, the first and second portions 50 and 60 are formed of silicon oxide (TEOS) and undoped polycrystalline silicon (which in the present context is considered to be insulating), respectively, other materials could be used, provided that the material used to form the second portion 60 can be etched selectively with respect to the material of the insulating first portion 50. Thus, for example, the first portion 50 may be a silicon oxide portion whilst the second portion 60 is a silicon nitride portion. If a material is available for forming the second portion 60 which can be easily etched with high selectivity relative to the first portion 50 and the semiconductor body then the further insulating layer 36 could, if desired, be omitted. Where the further insulating layer 36 is present, as in the example described above, beneath the second portion 60, the second portion 60 may be formed of a semiconductor or even conductive material, for example a doped polycrystalline silicon. also, dopable materials other than polycrystalline silicon, for example amorphous silicon, may be used for the doped layer 31 and layer 7.
In addition, a method embodying the invention may be applied to bipolar transistors which are not symmetrical about the emitter region 70, that is, for example, where the base contact B is provided on only one side of the emitter region and to lateral bipolar transistors in addition to the vertical type of bipolar transistor shown in Figure 8. Also, a method embodying the invention may be applied to other types of semiconductor devices. Of course, the conductivity types given above could be reversed and a method embodying the invention may be applied where the semiconductor body 1 is formed of a material other than silicon, for example a III-V material such as gallium arsenide.
It has been stated above that the drawings illustrate examples of embodiments of the invention and, in order to avoid any misunderstanding, it is hereby further stated that, in the following claims, where technical features mentioned in any claim are followed by reference signs relating to features in the drawings and placed between parentheses, these reference signs have been included in accordance with Rule 29(7) EPC for the sole purpose of facilitating of the claim, by reference to an example.

Claims

A method of manufacturing a semiconductor device having first, second and third regions, which method comprises:
providing a semiconductor body (10) having in one major surface (11) the first region (20) having a first conductivity type;

providing on the one major surface (11) a layer (30) doped with impurities of the opposite conductivity type, and forming an opening (31) through the doped layer (30) over the first region (20);

causing impurities to diffuse from the doped layer (30) into the semiconductor body (10) thus forming an extrinsic subsidiary region (41) of the second region (40) within the first region (20) said extrinsic subsidiary region (41) thus being the opposite conductivity type;

introducing through the opening (31) impurities to form a coupling region (43) of the opposite conductivity type within the first region (20) and joining with the extrinsic subsidiary region (41);

providing a first layer (5) of insulating material over the opening (31), and etching the insulating material to leave first portions (50) of the insulating material on the side walls (32) of the doped layer (30) bounding the opening (31) so that the first portions (50) define a first window (80) smaller than the opening (31);

introducing impurities for forming an intrinsic subsidiary region (42) of the second region (40) through the first window (80) said second region (40) thus comprising the extrinsic subsidiary region (41), the coupling region (43) and the intrinsic subsidiary region (42) with the coupling region (43) joining the extrinsic subsidiary region (41) to the intrinsic subsidiary region (42);

providing a second layer (6) of a material different from the first layer (5) over the first window (80);

etching the second layer (6) selectively with respect to the first portions (50) to define on the insulating first portions (50) second portions (60) of the different material, which second portions (60) define a second window (90) smaller than the first window (80); and

introducing impurities of the first conductivity type through the second smaller window (90) to form the third region (70) within the intrinsic subsidiary region (42).
A method according to Claim 1, further comprising implanting the impurities for forming the coupling region (43) using a dose and energy such that the coupling region (43) is more shallow and lowly doped than the intrinsic subsidiary region (42) of the second region (40).
A method according to Claim 1 or 2, further comprising defining the insulating first portions (50) by anisotropically etching the insulating first layer (5) provided over the opening and by defining the second portions (60) by anisotropically etching the second layer (6) selectively with respect to the insulating first portions (50).
A method according to Claim 3, further comprising providing a further insulating layer (36) over the first window (80) prior to the second layer (6) and anisotropically etching the second layer (6) selectively with respect to the further insulating layer (36).
A method according to Claim 1, 2, 3 or 4, further comprising providing the second portions (60) as insulating material portions.
A method according to any one of the preceding claims, further comprising providing the first and second portions (50 and 60) as portions of silicon oxide and polycrystalline silicon, respectively.
A method according to any one of the preceding claims, further comprising introducing the impurities to form the third region (70) by providing a layer (7) doped with impurities of the first conductivity type in the second window (90).
A method according to any one of the preceding claims, further comprising defining the first, second and third regions (20, 40 and 70) as the collector, base and emitter regions, respectively, of a bipolar transistor.